1. Field of the Invention
The present invention relates to an antenna apparatus for transmitting and receiving analog radio frequency signals or digital signals in a microwave band, a millimeter-wave band, etc. More particularly, the present invention relates to a slot antenna apparatus operable in a wideband with a constant beam width.
2. Description of the Related Art
A wireless device operable in a much wider band than that of prior art devices is required for the following two reasons. As the first reason, it is intended to implement a novel short-range wireless communication system with the authorization of use of a very wide frequency band, i.e., an ultra-wideband (UWB) wireless communication system. As the second reason, it is intended to utilize a variety of communication systems each using different frequencies, by mans of one terminal.
For example, when converting a frequency band into a fractional bandwidth being normalized by a center frequency fc of an operating band, a frequency band from 3.1 GHz to 10.6 GHz authorized for UWB in U.S. corresponds to a value of 109.5%, indicating a very wide band. On the other hand, in cases of a patch antenna and a one-half effective wavelength slot antenna which are known as basic antennas, the operating bands converted to fractional bandwidths are less than 5% and less than 10%, respectively, and thus, such antennas can not achieve a wideband property such as that of UWB. For example, referring to the frequency bands currently used for wireless communications in the world, a fractional bandwidth to the extent of 30% should be achieved in order to cover bands from the 1.8 GHz band to the 2.4 GHz band with one same antenna, and similarly, a fractional bandwidth to the extent of 90% should be achieved in order to simultaneously cover the 800 MHz band and the 2 GHz band with one same antenna. Furthermore, in order to simultaneously cover bands from the 800 MHz band to the 2.4 GHz band, a fractional bandwidth of 100% or more is required. The more the number of systems simultaneously handled by one same terminal increases, thus resulting in the extension of a frequency band to be covered, the more a wideband antenna with small size is required to be implemented.
A one-end-open one-quarter effective wavelength slot antenna is one of the most basic planar antennas, and a schematic view of this antenna is shown in FIGS. 33A, 33B, and 33C (hereinafter, referred to as a “first prior art example”). FIG. 33A is a schematic top view showing a structure of a typical one-quarter effective wavelength slot antenna (showing a grounding conductor 103 on a backside by phantom), FIG. 33B is a schematic cross-sectional view along the dashed line in FIG. 33A, and FIG. 33C is a schematic view showing a structure of the backside of the slot antenna in FIG. 33A by phantom. As shown in FIGS. 33A, 33B, and 33C, a feed line 113 is provided on a front-side of a dielectric substrate 101, and a notch with a width Ws and a length Ls is formed in a depth direction 109a from an outer edge 105a of an infinite grounding conductor 103 provided on a backside thereof. The notch operates as a slot resonator 111, one of its ends is opened at an open end 107. The slot 111 is a circuit element which is obtained by completely removing a conductor in thickness direction, in a partial region of the grounding conductor 103, and which resonates near a frequency fs at which one-quarter of the effective wavelength is equivalent to the slot length Ls. The feed line 113 formed in a width direction 109b intersects with the slot 111 at a portion thereof, and electromagnetically excites the slot 111. A connection to an external circuit is established through an input terminal. Note that according to common practice, a distance Lm of the feed line 113 from its open-ended termination point 119 to the slot 111 is set to the extent of one-quarter effective wavelength at the frequency fs, so as to achieve input impedance matching. Further, note that according to common practice, a line width W1 is designed based on a thickness H of the substrate and a permittivity of the substrate, such that the characteristic impedance of the feed line 113 is set to 50 Ω.
As shown in FIGS. 34A, 34B, and 34C, Patent Document 1 discloses a structure for operating the one-quarter effective wavelength slot antenna shown in the first prior art example, at a plurality of resonant frequencies (hereinafter, referred to as a “second prior art example”). A slot 111 has a slot length Ls, and includes a capacitor 16 so as to connect points 16a and 16b each located a distance Ls2 away from an open end. When the antenna is excited at a plurality of resonant frequencies at a feeding point 15, the antenna operates with different slot lengths Ls and Ls2 as shown in FIGS. 34B and 34C, and thus the bandwidth can be extended. However, according to the frequency characteristics shown in Patent Document 1, it is not enough to obtain a currently required ultra-wideband characteristics.
Non-Patent Document 1 discloses a method of operating a slot resonator in a wideband, which is short-circuited at both ends of a slot, and is of a one-half effective wavelength slot antenna (hereinafter, referred to as the “third prior art example”). FIG. 35is a schematic top view showing a structure of a slot antenna described in Non-Patent Document 1. In FIG. 35, a grounding conductor 103 and a slot 111 on a backside of a substrate are shown by phantom. The slot 111 is formed in the grounding conductor 103, such that the slot 111 has a certain width Ws, and a length Ls equivalent to one-half effective wavelength, and such that the slot 111 is coupled to a feed line 113 at a position 51a which is offset by a distance d from the center of the slot 111. According to prior art methods for matching input impedance of a slot antenna, a method has been used in which for exciting the slot 111, the feed line 113 intersects with the slot 111 at a position on the feed line 113 apart from an open-ended termination point 119 by one-quarter effective wavelength at a frequency fs. However, as shown in FIG. 35, in the third prior art example, a region extending over a distance Lind from the open-ended termination point 119 of the feed line 113 is replaced by an inductive region 121 which is a transmission line with a characteristic impedance higher than 50 Ω, and that inductive region 121 is coupled to the slot 111 at substantially the center of the inductive region 121 (i.e., in FIG. 35, t1 and t2 are substantially equal to each other). In this case, a width W2 of the inductive region 121 is set to a certain width narrower than the width of the feed line 113, the length Lind of the inductive region 121 is set to one-quarter effective wavelength at a center frequency f0 of an operating band, and the inductive region 121 operates as a one-quarter wavelength resonator different from the slot resonator. As a result, an equivalent circuit structure includes two resonators, which is increased from one resonator that is included in a typical slot antenna, and a double-resonance operation is achieved by coupling the resonators resonating at frequencies close to each other. In an example shown in FIG. 2(b) of Non-Patent Document 1, a good reflection impedance characteristic of −10 dB or less is achieved at a fractional bandwidth of 32% (near 4.1 GHz to near 5.7 GHz). As shown in comparison of actual measurement results of reflection characteristics versus frequency in FIG. 4 of Non-Patent Document 1, the fractional bandwidth of the antenna of the third prior art example is much wider than a fractional bandwidth of 9% of a typical slot antenna fabricated under conditions using the same substrate.
Further, in Non-Patent Document 2 shown as a fourth prior art example, a printed monopole antenna as one type of monopole antennas, known by its wideband operation, is successfully operated with low reflection in the UWB band. However, as is clearly seen from an E-plane radiation pattern shown in FIG. 5(b) of Non-Patent Document 2, the main beam direction greatly changes depending on frequency. In addition, the half-width of the main beam in the E-plane also greatly varies depending on frequency.
Non-Patent Document 3 shown as a fifth prior art example reports the results of detailed analysis on current distributions for each operation mode, for the purpose of extending the operating band of a one-quarter effective wavelength slot antenna. Non-Patent Document 3 asserts that by adding a grounding conductor in a stub form to the center of a slot such that the slot is split in two in a width direction, it is possible to suppress a non-radiative current distribution mode, thus extending the operating band.
Prior art documents related to the present invention are as follows:
(1) Patent Document 1: Japanese Patent Laid-Open Publication No. 2004-336328;
(2) Non-Patent Document 1: L. Zhu, et al., “A Novel Broadband Microstrip-Fed Wide Slot Antenna With Double Rejection Zeros”, IEEE Antennas and Wireless Propagation Letters, Vol. 2, pp. 194-196, 2003;
(3) Non-Patent Document 2: H. R. Chuang, et al., “A Printed UWB Triangular Monopole Antenna”, Microwave Journal, Vol. 49, No. 1, January 2006; and
(4) Non-Patent Document 3: M. Cabedo-Fabrés, “Wideband Radiating Ground Plane with Notches”, IEEE Antennas and Propagation International Symposium, pp. 560-563, 2005.
As discussed above, sufficient wide band operation has not been achieved in the prior art slot antennas. In addition, in a printed monopole antenna which is expected as a wideband antenna for UWB, it is difficult to maintain the main beam direction across an operating band, and it is also difficult to maintain the half-width of the main beam in an E-plane across the operating band. As a result, even when such an antenna is applied to a UWB system, it is difficult to efficiently cover one same area.
First of all, in the case of a typical one-end-open slot antenna with only one resonator in its configuration as in the first prior art example, a frequency band, where a good reflection impedance characteristic can be achieved, is limited to a fractional bandwidth to the extent of a little less than 10%.
In the second prior art example, although a wideband operation is achieved by incorporating a capacitive reactance element into a slot, it can be readily noticed that additional components such as a chip capacitor are required, and the characteristics of the antenna vary depending on variations in characteristics of the newly incorporated additional components. Further, according to the examples disclosed in FIGS. 13 and 19 of Patent Document 1, it is difficult to achieve a characteristic of input impedance matching with low reflection in an ultra-wideband.
In the third prior art example, the fractional bandwidth characteristic is limited to the extent of 35%. Further, as compared to the antennas of the first and second prior art examples with one-end-open slot resonators which are of one-quarter effective wavelength resonators, it is disadvantageous in reducing size to use a slot resonator which is short-circuited at both ends and is of a one-half effective wavelength resonator.
In the fourth prior art example, although the low-reflection characteristic is achieved over the entire UWB band, the radiation characteristics considerably vary in the band. Referring to a radiation pattern diagram in FIG. 5(b) of Non-Patent Document 2, the gain in a 225-degree direction decreases by 6 dB at 5 GHz, and by as much as 15 dB at 7 GHz, as compared to a reference gain value at 4 GHz. When such gain variations occur, it becomes extremely difficult to stably establish communication conditions over the entire band. Further, since the half-width of the main beam varies depending on frequency, it can not be considered that the communication area is being efficiently covered.
According to the fifth prior art example, although it is asserted that the operating band of an unbalanced-feed one-quarter effective wavelength slot antenna is extended, reflection intensity is high over the entire band, and thus, the extension of the band can not be considered to be achieved. Further, the fifth prior art example does not mention radiation characteristics.